5.5.9. Limitations of Online DDL

Take the following considerations into account when creating or dropping InnoDB indexes:

During an online DDL operation that copies the table, files are written to the
temporary directory ($TMPDIR on Unix, %TEMP%
on Windows, or the directory specified by the --tmpdir configuration variable). Each temporary file is large enough
to hold one column in the new table or index, and each one is removed as soon as it is merged into the
final table or index.

An ALTER
TABLE statement that contains DROP INDEX and ADD INDEX clauses that both name the same index uses a table copy, not Fast
Index Creation.

The table is copied, rather than using Fast Index Creation when you create an index
on a TEMPORARY TABLE. This has been reported as MySQL Bug #39833.

The ALTER TABLE
clause LOCK=NONE is not allowed if there are ON...CASCADE
or ON...SET NULL constraints on the table.

During each online DDL ALTER
TABLE statement, regardless of the LOCK clause, there are brief
periods at the beginning and end requiring an exclusive lock on the table
(the same kind of lock specified by the LOCK=EXCLUSIVE clause). Thus, an
online DDL operation might wait before starting if there is a long-running transaction performing
inserts, updates, deletes, or SELECT ... FOR UPDATE on that table; and an
online DDL operation might wait before finishing if a similar long-running transaction was started while
the ALTER TABLE was in progress.

OPTIMIZE
TABLE for an InnoDB table is mapped to an ALTER TABLE operation to rebuild the table and update index statistics
and free unused space in the clustered index. This operation does not use fast index creation. Secondary
indexes are not created as efficiently because keys are inserted in the order they appeared in the
primary key.